The present invention relates to a process and a device for processing carbon fiber strands.
For producing semi-finished carbon fiber products, so-called rovings are used, which are fiber strands, fiber bundles or multi-filament yarns consisting of several thousand or several tens of thousands of filaments (continuous fibers) arranged parallel or with a slight twist (false twist for preventing a coming-apart), which are traded on spools, rolls or drums and are continuously pulled off for the processing. These are called on-line methods in contrast to the discontinuous manual placing. The diameter of each individual filament is usually between 5 and 8 μm.
In the case of the on-line processing of carbon fiber strands, it is often necessary to heat up the starting product (the carbon fiber) and partially raise it to high temperatures. It is known to accomplish the heating by the use of furnaces, Bunsen burners, heat lamps or other radiation sources. The fiber is guided through the heat source at the production speed, in which case the heating of the fiber material is to be set by varying the temperature and the speed. However, in these processes, the heating-through of the fiber material is often not sufficiently homogeneous. This may result in fluctuations along the manufacturing process and thereby in differences in the product characteristics of the semi-finished product. The heat input may also be too spotty or too intensive on the whole, which may damage the fibers. Because of high heat losses, the energy efficiency of the above-mentioned methods may also not be sufficient.
It is an object of the present invention to provide a method and a device for processing carbon fiber strands which at least partially eliminates the disadvantages of the prior art. In particular, it is an object of the present invention to provide an easily controllable method and a device for processing carbon fiber strands, which permits a homogenous and smooth heating-through of the fiber material in a carbon fiber strand, so that fiber damage can be avoided. It is a further object of the invention to reduce the energy expenditures for the heating of the fiber material.
At least in partial aspects, the above-mentioned object is achieved by a method and a device according to embodiments of the invention. The characteristics and details described in connection with the device according to the invention also apply to the system according to the invention, to the facility according to the invention, and to the method according to the invention and, in each case, vice-versa and alternately, so that, with respect to the disclosure, reference is made or can be made, always alternatively, to the individual aspects of the invention.
A first aspect of the present invention relates to a method of heating a continuously conveyed carbon fiber strand. According to the invention, the heating takes place by feeding electric current into the carbon fiber strand.
A carbon fiber strand is a strand of untwisted or only minimally twisted quasi-continuous filaments of carbon. When the heating of the carbon fiber strand takes place by feeding electric current into the carbon fiber strand, the temperature control of the filaments takes place from inside the material, so that heat can be fed into the carbon fiber strand in a uniform and homogenous and, therefore, smooth manner. The temperature gradient in the fiber is inverse to a heating from the outside.
When the carbon fiber strand is spread open before the feeding of electric current, a contacting of the carbon fiber strand can be improved because the fiber filaments are distribute over a broad area.
In a preferred embodiment, the heating takes place to a temperature which corresponds at least to a softening temperature of a coating or impregnation situated on fibers of the carbon fiber strand. When the coating of the fibers is present in a softened (therefore particularly in a molten) condition, a subsequent production of composite parts will be facilitated because the coating may, for example, contain a matrix material for the fiber composite.
In a particularly preferred embodiment, the heating takes place to a temperature which corresponds at least to a disintegration temperature of a coating situated on fibers of the carbon fiber strand. As a result, a coating present in the delivery condition can be removed if the subsequent processing steps require no or a different coating.
In a preferred further development, a final temperature of the carbon fiber strand achieved by the heating will be controlled or automatically controlled by at least one of the following measures:                Varying of a voltage at which the electric current is fed;        varying of a protective resistor;        varying of a withdrawal speed of the carbon fiber strand;        varying of a spacing of current feeding points.        
This results in a simple controllability.
A further aspect of the present invention relates to a heating device for heating a continuously conveyed carbon fiber strand. The heating device according to the invention is designed for the implementation of the above-described method.
In a preferred embodiment, the heating device has a voltage source and at least two contact elements connected with respective poles of the voltage source and insulated from the environment. The contact elements are designed for contacting the carbon fiber strand such that a closed circuit is formed with the voltage source. The contact elements may have a contact roller and/or a sliding contact. According to the invention, a contact roll may also be understood to be a contact roller. Depending on the requirement, the contact roller may have a convex or concave design.
In particular, the heating device has a control unit which is designed for triggering the voltage source.
In a preferred further development, a temperature sensor is provided for measuring a final temperature of the carbon fiber strand. The control unit is designed for receiving an output signal of the temperature sensor and for automatically controlling the final temperature of the carbon fiber strand by applying at least one of the following measures:                Triggering the voltage source in order to vary an output voltage of the voltage source;        triggering a variable resistor in order to vary a voltage between the contact elements;        triggering a servo drive in order to vary the spacing of contact elements;        triggering a driving device in order to vary the withdrawal speed of the carbon fiber strand.        
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.